Allow negative amounts in any category for refunds and paybacks

[emavgl]

Description

Currently, Oinkoin forces the sign of a record's value based on its category type:

• Expense category → value is always stored as negative (e.g., -50€)
• Income category → value is always stored as positive (e.g., +100€)

This prevents users from recording refunds or paybacks in the same category where the original transaction occurred. For example:

• A 10€ refund on groceries cannot be entered in the "Groceries" expense category (the app turns it into -10€, making expenses look even larger)
• A 100€ payback of income (employer mistake) cannot be entered as negative income

The workaround (creating a separate "Refunds" income category) inflates statistics — showing more expenses AND more income than reality.

Requested Behavior (inspired by "Mein Haushaltsbuch")

Users should be able to enter negative amounts in any category:

• -10€ in an expense category → reduces that category's expense total (net expense of -40€ instead of separate -50€ expense + 10€ refund in another category)
• -100€ in an income category → reduces income totals
• Statistics show realistic net totals per category

Technical Analysis

No database changes needed

The records table stores value REAL with no CHECK constraints (lib/services/database/sqlite-migration-service.dart:37). The sign of the value is not enforced at the DB level.

What needs to change

1. lib/records/edit-record-page.dart (line 967-989):

   • Change allowNegative: false → allowNegative: true on the AmountInputField
   • Remove the forced sign assignment in changeRecordValue() — stop calling abs() and re-negating based on category type. Let the user's entered sign be stored as-is.

2. lib/components/amount_input_field.dart — Already supports allowNegative: true (line 24), no changes needed.

3. lib/helpers/records-utility-functions.dart — The parsers tryParseCurrencyString and tryParseSignedCurrencyString already exist (lines 213-220).

Statistics impact

Most aggregation code groups records by CategoryType and sums values directly, which handles refunds correctly:

• records_per_day.dart — sums all values for a given CategoryType → refunds naturally reduce totals ✓
• statistics-page.dart — filters by CategoryType → refunds in expense categories show up in the Expenses tab ✓
• balance-chart-models.dart — uses value.abs() → may need review for records with "wrong" signs

Some edge cases in lib/models/records-per-category.dart — uses value < 0 / value > 0 to split income/expense within a category. A positive refund in an expense category would show as "income" in the per-category breakdown rather than reducing expenses. This may need updating.

Files Likely Affected

• lib/records/edit-record-page.dart — Remove sign enforcement
• lib/models/records-per-category.dart — Review sign-based splitting
• lib/statistics/balance-chart-models.dart — Review value.abs() usage
• lib/records/controllers/tab_records_controller.dart — Review applyTransferAwareWalletFilter for edge cases

[emavgl]

Implemented on branch feature/allow-negative-amounts-refunds.

Approach: Records keep the user-entered sign as-is. All aggregations sum the raw (signed) values and only take the magnitude of the total (abs(sum(raw))), never a per-record abs(). So a refund recorded as +10 in an expense category nets -40 against -50, and a payback recorded as -100 in an income category nets +900 against +1000. Charts show the magnitude of each slice/period's net.

Changes:

• edit-record-page.dart: allowNegative: true on the amount field; value is parsed with tryParseSignedCurrencyString and stored without re-signing on category switch; edit/recurrent fields show the signed value.
• records-per-category.dart: income/expenses split by category type (sum raw), not by sign.
• records-utility-functions.dart: buildCurrencyBreakdown, computeConvertedTotal, computeTotalInCurrency now net refunds (abs applied to the total when isAbsValue).
• statistics-utils.dart / statistics-calculator.dart: period aggregation sums raw values; avg/median take magnitudes of the totals in non-balance mode.
• balance-chart-models.dart: ComparisonDataAggregator nets expenses/income per period.
• Pie charts (categories/tags/wallets): each slice is abs(net).

Tests: 25 new cases across 4 files covering the edit page, RecordsPerCategory, aggregation helpers, and statistics (avg/median/balance chart/RecordsPerDay). Full suite passes: 848 tests.

Backward compatible — for existing all-negative (expense) / all-positive (income) data the results are identical since abs(sum(raw)) == sum(abs(raw)) when signs are consistent.

Deferred (out of scope): CSV import's sign→category-type inference in csv_import_service.dart.